For many applications it is necessary to provide a device for measuring the time interval between first and second events. If the first and second events are time spaced from each other by an extremely short duration, such as less than one nanosecond or on the order of a few nanoseconds, there are manifold problems in determining the interval between the events.
Prior art devices have been developed to determine the time interval between first and second events spaced from each other on the order of a nanosecond. In one device analog interpolators measure and reduce a .+-.1 count ambiguity. In this technique, three separate measurements are made to determine the interval between a start pulse (first event) and a stop pulse (second event). The first measurement determines a first time interval between the first event and the first reference clock pulse that occurs after the first event and a second interval between the second event and the first reference clock pulse which occurs after the second event. The first and second time intervals are determined by stretching the intervals in separate analog pulse stretcher networks. The pulse stretchers derive pulses having widths that are linearly proportional to the time separating the pulses applied to them. The total time interval between the first and second events is determined by adding together the first two measurements and subtracting the resultant sum from the third measurement.
In a second technique, described fully in the Hewlett-Packard Journal for August, 1978, and referred to as Dual Vernier Interpolation, two triggered phase-lock oscillators are responsive to the first and second events. Output signals from the two triggered oscillators are combined. Extensive computations are performed by circuitry responsive to the combined signals from the triggered oscillators to derive the desired time interval.
Both of the prior art structures are relatively bulky and expensive and employ methods requiring stretching of a time base. This prevents multiple stop pulses from being received prior to completion of the interpolation time. Thus, the prior art devices are not suitable to many systems in which a last return for stop pulse timing is required. The duty cycle of the prior art systems is limited by the interpolation time of the circuitry included therein; because the interpolation time is normally on the order of a milisecond, the prior art systems are not suitable to detect multiple events that occur more frequently than on the order of a millisecond. The prior art devices also consume a relatively great amount of power and require analog components, rather than standard, available digital components.
It is, accordingly, an object of the present invention to provide a new and improved apparatus for determining the interval between first and second events.
Another object of the invention is to provide a new and improved apparatus for determining the interval between first and second events which may occur on the order of a nanosecond from each other.
Another object of the invention is to provide a new and improved relatively low volume, low power consuming device using standard, available digital components for determining the interval between first and second events that can occur on the order of a nanosecond or less relative to each other.
A further object of the invention is to provide a new and improved apparatus for determining the interval between first and second events that occur within each other on the order of a nanosecond, wherein a direct indication of the time interval is provided without making computations.